Lubricating compositions generally comprise a base oil of lubricating viscosity together with one or more additives to deliver properties including for example, reduced friction and wear, improved viscosity index, detergency, and resistance to oxidation and corrosion. A lubricant base oil may comprise one or more lubricating base stocks.
Lubricant base stocks used in automotive engine lubricants are generally obtained from petrochemical sources, for example they may be obtained as the higher boiling fractions isolated during the refining of crude oil or as the products of chemical reactions of feedstocks from petrochemical sources. Lubricant base stocks can also be made from Fischer-Tropsch wax.
Lubricant base stocks may be classified as Group I, II, III, IV and V base stocks according to API standard 1509, “ENGINE OIL LICENSING AND CERTIFICATION SYSTEM”, 17th Edition, Annex E (October 2013 with Errata March 2015), as set out in Table 1.
TABLE 1SaturatedSulphur contenthydrocarbon(% by weight)content (%ASTM D2622,by weight)D4294, D4927,ASTMD3120 orViscosity IndexGroupD2007D1552ASTM D2270I<90and/or>0.03and≥80 and <120II≥90and≤0.03and≥80 and <120III≥90and≤0.03and≥120IVPolyalphaolefinsVall base stocks not in Groups I, II, III or IV
Group I base stocks are typically manufactured by known processes including, for example, solvent extraction and solvent dewaxing, or solvent extraction and catalytic dewaxing. Group II and Group III base stocks are typically manufactured by known processes including, for example, catalytic hydrogenation and/or catalytic hydrocracking, and catalytic hydroisomerisation. Group IV base stocks include for example, hydrogenated oligomers of alpha olefins.
A combination of properties is desirable in a base stock. In some instances, for example in passenger car engine oils, it may be desirable for a base stock to have a low viscosity profile, since this leads to improved fuel economy. In particular, it is desirable for base stocks to have a low kinematic viscosity as well as good low-temperature viscosity characteristics, for example a low pour point or low viscosity as measured using a mini-rotary viscometer (MRV). However, the general trend is for an improvement in the viscosity profile (i.e. a reduction in viscosity parameters) of a base oil to be accompanied by an undesirable increase in volatility.
Accordingly, there is a need in the art for a base stock having a desirable viscosity profile, including good low-temperature viscosity characteristics, but which also exhibits low volatility.
Problems may also be encountered when a base stock is incorporated into a lubricating composition and used in an engine. For instance, poor miscibility of a base stock with lubricant additives or other base stocks may lead to problems in the engine, for instance with piston cleanliness. Negative interactions between a base stock and oil seals that are found in engines may, in some cases, lead to loss of lubricant through failure of the oil seals. Base stocks may also undergo oxidative degradation at the high temperatures encountered in an engine. Base stocks containing polar groups such as ester or other groups may be particularly prone to at least some of these problems.
Accordingly, there is a need for a base stock having low volatility for a given viscosity profile, but which is also suitable for use, for example, in a lubricating composition for an internal combustion engine.